Portable radio frequency (RF) transmitting devices, including cellular phones, portable radios, wireless modems, wireless routers, blue-tooth devices, and the like, are often energized by batteries. Currently, lithium-ion (Li-ion), nickel-cadmium (NiCd), nickel metal hydride (NiMH), and alkaline batteries in configurations that produce an operational voltage in the range of 3-7 Vdc when fully charged are popular for use in portable RF transmitting devices.
Portable RF transmitting devices are often mass produced for competitive markets. In other words, a vast multiplicity of a given make and model of an RF transmitting device is likely to be manufactured and widely distributed to the consuming public as inexpensively as possible. Accordingly, a need exists for RF transmitting devices that are easily compatible with the popular styles of batteries used in RF transmitting devices, inexpensive, yet robust and reliable.
In an attempt to meet this need, mass market RF transmitting devices tend to incorporate integrated circuits that include as many different functions on a single IC as practical and are manufactured using processes that achieve reliable results at a high yield. One such process is a complementary metal oxide semiconductor (CMOS) process. Standard CMOS processes are highly desired for use in forming RF transmitting device circuits because the resulting circuits tend to be reliable, robust, and relatively inexpensive.
One challenge of using standard CMOS processes for the formation of an RF amplifier portion of an RF transmitting device concerns the low breakdown voltages that characterize standard CMOS processes. In particular, low transistor breakdown voltages in the range of 2.8-3.6 volts routinely result from the adherence to standard CMOS processes. But this low breakdown voltage is not easily compatible with the batteries popularly used in portable RF transmitting devices. Additional circuits, techniques, and/or non-standard processes are conventionally used to accommodate the relatively high voltage delivered by the currently popular batteries. But these additional circuits, techniques and/or non-standard processes lead to increased costs.
A conventional RF amplifier configuration for a portable RF transmitting device may use Si bipolar, SiGe HBT, GaAs HBT, and/or other transistor formation processes. These transistor formation processes can result in higher breakdown voltages better matched to popular battery voltages. Unfortunately, each of these processes increases costs when compared to a standard CMOS process.
Costs are increased in at least two ways, transistor area and support circuitry, which dramatically drive up costs. For example, a large number of RF chokes (i.e., inductors) and large-valued bypass capacitors tend to be used in RF amplifiers that use these types of transistors for their active components. The excessive use of chokes and/or large bypass capacitors consumes precious semiconductor substrate area, leading to further increases in costs.
A conventional RF amplifier configuration for an RF transmitting device may alternatively use a metal oxide semiconductor (MOS) transistor for the active component of an RF amplifier, but limit its voltages using a voltage regulation circuit. This technique is also undesirable because the inclusion of a voltage regulator increases semiconductor substrate area and thereby increases costs. Moreover, a voltage regulator is likely to be an inefficient section which wastes power, and the wasting of power is highly undesirable in a battery-powered device.
Another technique conventionally used to adapt CMOS processes to a battery-powered, RF amplifier application stacks MOS transistors so that the available voltage is distributed across transistors in the stack.
One conventional version of this stacked-transistor technique forms a modified cascode amplifier, where a common gate transistor configuration is stacked with a common source transistor configuration, the common source transistor is driven with an RF input signal, and the gate biasing of the common gate transistor is modified so that the total voltage at DC is evenly distributed across the transistors. But this version suffers from undesirably low gain, undesirably low output impedance, and it fails to maintain an even distribution of voltage across the transistors. Low gain is a challenge when using any CMOS technique to form an RF amplifier, so the lower-gain amplification techniques are particularly unwelcome. The low gain and low impedance characteristics may be addressed using known techniques, but in addressing these characteristics even more semiconductor substrate area is consumed. And, to the extent that voltage distribution is uneven under RF conditions, the risk of catastrophic failure due to exceeding the breakdown voltage on the transistor with the greatest voltage increases.
Another conventional version of the stacked-transistor technique is particularly unsuited for a linear amplification application. This technique uses multiple inductors in the transistor stack and concurrently switches the different transistors using out-of-phase versions of the RF input signal. Resorting to the use of many inductors in an RF amplifier design is undesirable because inductors consume an inordinate amount of semiconductor substrate area. And, when many inductors are used the RF amplifier size increases considerably. Moreover, RF amplifiers that fail to operate in linear amplification applications are undesirable because many of the more modern modulation standards call for amplification to a high degree of linearity.